The present invention generally relates to a flow rate detecting device for detecting an instantaneous flow rate of a fluid, which is provided, for example, in a water flow passage for a water or hot water supplying apparatus, or in a fuel supplying passage and a combustion air supplying passage for a burning apparatus, and more particularly, to a flow rate for a burning apparatus, and more particularly, to a flow rate detecting device for detecting a flow rate as a mechanical action which is converted into electrical signals.
As one of the conventional flow rate detecting devices, means for detecting the flow rate directly by an electric signal have been constructed, but owing to the large size and high cost, they are generally used as electronic measuring instruments, and in the application to the public welfare appliances, there have often been utilized a means having a mechanical portion to be operated in proportion to the flow rate for detecting the operation by an electric signal.
In the construction illustrated in FIG. 1, the number of revolutions is detected by a permanent magnet 5 buried in a circumferential portion of a rotary blade 4, which is supported by an upstream side bearing 2 and a downstream side bearing 3 in a flow passage F, and a magnetic sensor 7 fitted on a fixed housing 1a.
Accordingly, the magnetic sensor 7 emits pulses proportional in number to the flow rate, since the rotary blade 4 is rotated in proportion to the flow rate.
In such a construction as mentioned above, it is apprehended that the the number of revolutions for a given flow rate may be reduced by fine sand particles or dust contained in a fluid, which has a possibility to enter the bearings 2 and 3 undesirably, on account of said bearings 2 and 3 being provided in the flow passage F, and it is also apprehended that the rotary blade 4 may become unrotatable due to the attachment of scraps or iron rust produced in piping works to permanent magnet 5, in the case of using iron pipes in an upstream side flow passage.
Another known example is such a flow rate detecting device as shown in FIG. 2, without any bearings in a flow passage.
In the conventional flow rate device of FIG. 2, there is provided a fixed blade 9 in the flow passage F, which turns a fluid into a rotary flow and an outflow path 12 at the downstream side thereof, through which the area of flow passage is annularly reduced in accordance with the construction of an inclined face 10 reducing said area inwards and an inclined face 11 protruding to upstream side at the center of the flow passage. The rotation of a spherical member 13 supported by said inclined faces 10 and 11 is detected by a light emitting element 15 and a light receiving element 16, provided in a fixed housing 1b. Accordingly, since the rotary speed corresponding to the flow rate can be obtained by the fixed blade 9, the spherical member 13 comes to rotate at the number of revolutions corresponding to the flow rate, and the light receiving element 16 produces pulse signals, the output of which is decreased every time the spherical member 13 intercepts the light from the light emitting element 15.
The above arrangement can avoid such an influence of foreign substances in a fluid, which is a cause of reduced signal response as aforesaid with reference to FIG. 1. However, the spherical member 13 receives a force caused by collision of a fluid and a force caused by a differential pressure upstream and downstream of the annular outflow path 12, in the direction to be pressed against the inclined faces 10 and 11, and moreover, a force toward the center of the rotary flow works on the inclined face 11 due to the decreasing pressure toward the central portion thereof and a centrifugal force works on the inclined face 10 by the rotation of said spherical member 13.
These forces cause a large frictional force against the rotary action, by pressing the spherical member 13 against a V-shaped groove formed by the inclined faces 10 and 11. Accordingly, the above known arrangement has such a problem that a detecting accuracy at a high rate cannot be obtained, since the proportional relation between the flow rate and the number of revolutions of the spherical member 13 is unstable, and thus, a high rotary speed cannot be achieved.